1. Field of the Invention
The present invention relates to a side-face incidence type photo detector in which incident light entering the photo detector from a side direction thereof is refracted and detected at the detection area. The present invention especially relates to a photo detector in which incident light is refracted at beveled face formed on at least one edge portion of a second face of a semiconductor substrate, whereby the light path of the incident light is diverted.
2. Description of the Related Art
For optical fiber communications, photo detectors which are sensitive to light of a long wavelength band of 1.3 xcexcm to 1.55 xcexcm are used. These photo detectors are pin photo diodes formed of a material selected from InGaAs/InP-type semiconductor materials. Such photo detectors are classified based on the direction from which the photo detector receives incident light. Front-face incidence type and rear-face incidence type photo detectors are generally used. In a front-face incidence type photo detector, incident light enters the semiconductor substrate through a face thereof on which the detection area is formed, i.e., a first principal face. In a rear-face incidence type photo detector, incident light enters the detector through a face which is opposite to the first principal face of the semiconductor substrate (i.e., through a second principal face). In such front-face incidence type and rear-face incidence type photo detectors, incident light enters from a direction substantially perpendicular to the semiconductor substrate.
On the other hand, side-face incidence type photo detectors also have been developed in which incident light enters through a side face of the photo detector. Such side-face incidence type photo detectors are especially advantageous over front-face incidence type and rear-face type photo detectors, in the cases where the photo detector is mounted on a base along with other elements. For example, in the case where the photo detector is provided with an optical fiber for supplying incident light, the photo detector is first bonded onto the base of a flat package, and then the optical fiber can be attached along a horizontal direction with regard to the photo detector. In the case where the light exiting from the rear of a semiconductor laser is monitored by a photo detector, the semiconductor laser and a principal face of the photo detector can be bonded onto the same base. In either case, the devices can be easily assembled.
Such side-face incidence type photo detectors have been implemented by employing so-called waveguide type pin photo diodes, or pin photo diodes having a refraction-based light path diversion means within the photo detector, so as to utilize their structural features to refract light entering the semiconductor substrate.
A pin photo diode having a refraction-based light path diversion means is described, for example, in Japanese Laid-open Publication No. 8-316506. FIG. 6 shows a cross-sectional view of such a conventional photo detector 600. With reference to FIG. 6, a conventional photo detector 600 includes on a first principal face 602 of an n-type InP semiconductor substrate 601, an n-type Inp buffer layer 603, a low concentration n-type InGaAs light absorbing layer 604, and a low concentration n-type InP window layer 605 in this order. A first diffusion area 606 is formed in a portion of the window layer 605, by means of diffusing a p-type impurity, e.g., Zn, in an island shape. A portion of the light absorbing layer 604 underlying the first diffusion area 606 functions as a photo detection area 607. The photo detection area 607 is electrically connected to a negative electrode 608 through the first diffusion area 606. The negative electrode 608 is formed on the first diffusion area 606, and a positive electrode 609 is formed on a second diffusion area 610. The second diffusion area 610 is provided to form the positive electrode 609 on the first principal face 602. The second diffusion area 610 is simultaneously formed by the diffusion of p-type impurity when the first diffusion area 606 is formed. Beveled face 612 is formed on at least one edge portion of the second principal face 611 of the semiconductor substrate 601 by an etching process. In a conventional photo detector 600 having such a structure, incident light 613 from the side of the semiconductor substrate 601 is refracted at the beveled face 612, and the refracted light 614 enters the photo detection area 607.
The conventional photo detector 600 is characterized by refracting the incident light 613 at the beveled face 612 so as to divert the light path of the incident light. Japanese Laid-open Publication No. 8-316506 describes that the (111) surface having an angle of 54.7xc2x0 with regard to the second principal face 611 is preferably used for the beveled face 612. This is because the beveled face 612 is required to have a particular angle with respect to the second principal face 611 as well as to have a smooth surface. In order to ensure that the beveled face 612 on the semiconductor substrate 601 meets such requirements, it is the most convenient to use a wet etching, which exposes a certain crystal face orientation. For manufacturing the photo detector 600, a semiconductor substrate whose principal face is the (001) surface is generally used as the semiconductor substrate 601. When a wet etching for exposing a crystal face orientation is applied to such a semiconductor substrate 601, the (111) surface is often exposed. The resultant beveled face 612 having the (111) surface form an angle of 54.7xc2x0 with regard to the second principal face 611. The incident light 614 entering the interior of the semiconductor substrate 601 is refracted at such a beveled surface 612 so as to form an angle of 25.7xc2x0 with regard to the second principal face 611.
As another conventional example, Japanese Laid-open Publication No. 11-307806 describes a pin photo diode having a refraction-based light path diversion means within the photo detector. FIG. 7 shows a cross-sectional view of a second conventional photo detector 700. With reference to FIG. 7, a conventional photo detector 700 includes on the first principal face 702 of an n-type InP semiconductor substrate 701, a low concentration n-type InGaAs light absorbing layer 703, and a low concentration n-type InP window layer 704 in this order. A diffusion area 705 is formed in the window layer 704, by diffusing a p-type impurity, e.g., Zn, in an island shape. A portion of the light absorbing layer 703 underlying the diffusion area 705 functions as a photo detection area 706. The photo detection area 706 is electrically connected to a negative electrode 707 through the diffusion area 705. The negative electrode 707 is formed on the diffusion area 705, and a positive electrode 709 is formed on a second principal face 708 of the semiconductor substrate 701. Beveled face 710 is formed on at least one edge portion of the second principal face 708 of the semiconductor substrate 701. In a conventional photo detector 700 having such a structure, the incident light 711 entering the semiconductor substrate 701 from the side thereof is refracted at the beveled face 710, and the refracted light 712 enters the photo detection area 706.
The conventional photo detector 700 is characterized by using the (112) surface as the beveled face 710. When the beveled face 710 is the (112) surfaces, the beveled face 710 forms an angle of 35.3xc2x0 with regard to the second principal face 708. The (112) surface can be exposed by an etching using a mixed solution containing hydrochloric acid and nitric acid. Specifically, if the mixed solution for the etching is prepared so as to have an about 5:1 to about 3:1 hydrochloric acid-nitric acid volume ratio, the beveled face 710 may have an angle of exactly 35.3xc2x0 with regard to the second principal face 708, and may have a specular surface. The incident light 712 entering the interior of the semiconductor substrate 701 is refracted at such a beveled face 710 at an angle of 41.0xc2x0 with regard to the second principal face 708.
In the photo detector 700 whose beveled face 710 is the (112) surface, the angle between the beveled face 710 and the second principal face 708 is smaller than in the case where the beveled face 612 is the (111) surface (as in the aforementioned photo detector 600), and the distance from the incidence side face to the center of the photo detection area 706 surface becomes shorter. Therefore, it is possible to reduce the chip size of the photo detector 700.
However, in manufacturing the first and second conventional photo detectors 600 and 700, having structures as described with reference to FIG. 6 and FIG. 7, respectively, the beveled face 612 or 710 on the second principal face should be correctly positioned with regard to the photo detection area 607 or 706 on the first principal face. This requires a double-side aligner, which performs a positioning process through visual observation of both faces of a wafer on which the photo detector structure 600 or 700 is to be formed. A double-side aligner, however, is not commonly employed as a manufacturing machine for semiconductor devices. Therefore, additional manufacturing equipment needs to be introduced for producing a photo detector having such a structure, increasing the manufacturing cost of the photo detector.
In order to easily divide the wafer into individual photo detectors (i.e., chips) after the manufacturing process of the wafer, the thickness of the semiconductor substrate is generally maintained as thin as 200 xcexcm or less. In the structure of the aforementioned photo detector 600 or 700 shown in FIG. 6 or FIG. 7, the second principal face needs to be processed so as to form the beveled face 612 or 710, after reducing the thickness of the semiconductor substrate 601 or 701. Specifically, the thin film wafer with a thickness of 200 xcexcm or less is subjected to a photo lithography process so as to form the beveled face 612 or 710 on the second principal face. If a double-side aligner employing a contact exposure method is used in this case, the wafer tends to break and/or crack. This reduces the product yield, again increasing the manufacturing cost.
In one aspect of the invention, a photo detector includes: a semiconductor substrate having a first principal face and a second principal face; a photo detection area formed on the first principal face of the semiconductor substrate; a negative electrode electrically connected with the photo detection area; a positive electrode; a beveled face formed at least one edge portion of the first principal face of the semiconductor substrate; and a reflector formed on the second principal face of the semiconductor substrate, wherein incident light from outside of the semiconductor substrate, enters the semiconductor substrate at a side face of the photo detector while being refracted at the beveled face, and thereafter is reflected from the reflector so as to reach the photo detection area.
In one embodiment of the invention, the first principal face of the semiconductor substrate is the (001) surface and the beveled face is the (112) surface.
In another embodiment of the invention, the reflector is a reflective mirror including a metal film.
In still another embodiment of the invention, the reflector functions as the positive electrode.
In still another embodiment of the invention, the first principal face of the semiconductor substrate further includes a light absorbing layer formed thereon, a portion of the light absorbing layer functioning as the light detection area.
In still another embodiment of the invention, the light absorbing layer further includes a window layer having a diffusion area, and the photo detection area is electrically connected to the negative electrode through the diffusion area.
In still another embodiment of the invention, a photo detector further includes a contact hole which reaches the first principal face of the semiconductor substrate through the window layer and through the light absorbing layer, wherein the positive electrode makes contact with at least a bottom of the contact hole.
In still another embodiment of the invention, the reflector is a reflective mirror including a laminated structure of a insulating film and a metal film, and the insulating film is disposed between the second principal face and the metal film.
In still another embodiment of the invention, the reflector is a wavelength filter in which a plurality of films having respectively different indices of refraction are laminated.
In still another embodiment of the invention, the reflector is a diffraction grating.
In still another embodiment of the invention, the photo detector includes a plurality of photo detection areas and a plurality of negative electrodes corresponding to the plurality of photo detection areas, respectively.
In another aspect of the invention, a method for manufacturing a photo detector includes the steps of: forming a photo detection area on a first principal face of a semiconductor substrate having the first principal face and a second principal face; forming a negative electrode electrically connected to the photo detection area; forming a positive electrode; forming a beveled face on at least one edge portion of the first principal face of the semiconductor substrate; forming a reflector on the second principle face of the semiconductor substrate.
In one embodiment of the invention, the step of forming the reflector includes a step of forming the positive electrode, and the reflector functions as the positive electrode.
In another embodiment of the invention, a method for manufacturing a photo detector further includes a step of forming a light absorbing layer on the first principal face of the semiconductor substrate, wherein a portion of the light absorbing layer functions as a photo detection area.
In still another embodiment of the invention, a method for manufacturing a photo detector further includes a step of forming a window layer having a diffusion area, wherein the photo detection area is electrically connected to the negative electrode through the diffusion area.
In still another embodiment of the invention, a method for manufacturing a photo detector further includes a step of forming a contact hole reaching the first principal face of the semiconductor substrate through the window layer and through the light absorbing layer, wherein the positive electrode is formed so as to make contact with at least a bottom of the contact hole.
In still another embodiment of the invention, the step of forming a beveled face includes: depositing an insulating film so as to cover the negative electrode: forming an opening in a predetermined area by etching the insulating film,: etching the semiconductor substrate corresponding the predetermined area, by using the insulating film having the opening as a mask; and removing the insulating film.
In still another embodiment of the invention, the step of depositing the insulating film includes a step of depositing the insulating film so as to cover exposed surfaces of the positive electrode and the window layer; and the step of forming the beveled face includes a step of etching the window layer and the light absorbing layer corresponding to the predetermined area.
In still another embodiment of the invention, the semiconductor substrate includes InP.
In still another embodiment of the invention, the first principal face is the (001) surface and the step of forming the beveled face is conducted by etching using a mixed solution containing hydrochloric acid and nitric acid.
In still another embodiment of the invention, the volume ratio of the hydrochloric acid and the nitric acid is in the range of 5:1 to 3:1.
Thus, the invention described herein makes possible the advantage of providing a side incidence type photo detector in which the increase of manufacturing cost is effectively alleviated.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.